JPH02306469A - Drop-out detecting circuit - Google Patents

Abstract

PURPOSE:To obtain a drop-out detecting circuit which has no answer to a noise band, etc., in a high speed searching state by comparing the output of a limiter amplifier with the output of an integrating circuit and detecting the drop-out based on the comparison level. CONSTITUTION:The envelope detection output is converted by a limiter amplifier 4 so that the state of the output is changed between the H and L levels. Thus the deciding action of a 1st comparator 6 is stabilized. The reference voltage of a 20% level is supplied to a 2nd comparator 7. When the output of the amplifier 4 drops less than the level of the reference voltage, an H level is outputted. The level of this output is decided via an integrating circuit 5 so that the presence/absence of a video signal can be decided. Thus it is possible to obtain a drop-out detecting circuit which has no answer to a noise band, etc., in a high speed searching state.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to a dropout detection circuit for playback devices such as video tape recorders and video disc players.
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(b) An example of conventional technology is JP-A-58-222682 (HO4N5
/94) has a video tape recorder (VTR)
There is a disclosure of a configuration in which envelope detection is performed on the playback output (output from a rotating head) and dropout is detected based on the level.

(c) Problems to be Solved by the Invention In this method of detecting a drop in the level of the envelope, there is a drop in the level in the noise band part during high-speed playback, or in the first half or the second half due to contact between the tape and the head. It also responds to level drops that require a long connection time, such as a drop in the level of . Therefore, if dropout compensation is performed using a dropout compensation circuit using an IH delay circuit, the screen will distort and look unnatural (because 18 minutes of video signals are used repeatedly during the level drop period). ).

(d) Means for Solving the Problems The present invention proposes a dropout detection circuit that does not respond to a drop in level for a predetermined period of time or longer. Therefore, a conversion circuit that converts the playback output from the head into a binary signal according to the level, a delay circuit that delays the change of the output of the conversion circuit from at least one state to the other, and the conversion circuit and the delay circuit are provided. A comparison circuit is provided that compares the outputs of the circuits and outputs a dropout detection signal during a period when the output of the conversion circuit is in the other state and the output of the delay circuit is not in the other state.

(e) Effect Since the above-described configuration is adopted, even when the level of the reproduced output decreases, if the period determined by the delay circuit is exceeded, the dropout detection state will not occur.

(f) Example Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment, FIG. 2 is a characteristic diagram of the limiter amplifier, and FIG. 3 is a waveform diagram.

In Figure 1, 1 is an input terminal for the playback output from the rotary head, 2 is an AGC circuit that controls the envelope level of this playback output to be constant, 3 is an envelope detection circuit, and 4 is a limiter amplifier (DC amplifier). , 5 is a delay (integration) circuit, 6 is a first comparator, and 7 is a second comparator. 8 is a dropout detection signal output terminal;
9 is a playback signal presence/absence detection signal output terminal.

The limiter amplifier 4 has input/output characteristics shown in FIG. Yeah. Ah, AGC... Wow, set the rope level to 1.
When it is set to 00%, if there is an input with a level of approximately 25% or more, it will be saturated and output an H level. When the input level is approximately 25% or less, an output proportional to the input can be obtained.

The delay circuit 5 has a function of delaying the fall of the limiter amplifier 4 output. Specifically, it is an integrating circuit made up of a capacitor and a resistor. A diode is connected in parallel to the resistor, and a time constant determined by the resistor and capacitor is given to the fall of the output of the limiter amplifier 4, and the change is immediately transmitted when the output rises. I'm trying to make it happen.

The first comparator 6 compares the output of the limiter amplifier 4 and the output of the delay circuit 5, and outputs an H level only when the output level of the delay circuit 5 is high, thereby preventing dropout from occurring in the dropout compensation circuit (not shown). Indicates that there is a

The operation at this time will be explained in more detail with reference to FIG.

In the case of a relatively short dropout caused by dust on the tape, the output of the limiter amplifier 4 changes as shown by the solid line in (A). ``Correspondingly, the output of the integrating circuit 5 becomes as shown by the broken line in (A). Therefore, a dropout detection output as shown in (b) is obtained.

On the other hand, in the case of a long-term output drop such as in a noise band during a search, the output of the limiter amplifier 4 and the output of the integrating circuit 5 become as shown in the solid line and broken line (c), respectively. Therefore, the dropout detection signal becomes as shown in (d), and even if the level drop period is long, the period for instructing dropout is within the period determined by the integrating circuit 5.

That is, by converting the envelope detection output so that the state changes to approximately two values of H level and L level by the limiter amplifier 4 (therefore, a comparator can also be used. However, a limiter amplifier has a simpler configuration. ), the discrimination operation of the first comparator 6 is stabilized (because the level of the envelope detection output fluctuates)
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The second comparator 7 is supplied with a reference voltage of 20% level as shown in FIG. 2, and outputs an H level when the output of the limiter amplifier 4 becomes lower than this level. Therefore, by determining the level of this output via an integrating circuit, it is possible to determine the presence or absence of a video signal.

Deciding to what extent dropout should be determined is
It is determined by the input level to the integrating circuit 5 and the time constant of the integrating circuit 5, and in the embodiment, it is determined to be about 20H.

(g) Effects of the Invention According to the present invention as described above, it is possible to realize a dropout detection circuit that does not detect noise bands and the like during high-speed search, and the effect is great.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a characteristic diagram of a limiter amplifier, and FIG. 3 is a waveform diagram. 3...Envelope detection circuit, (4)...Limiter amplifier (conversion circuit), (5)...Integrator (delay) circuit,
(6) Comparator (comparison circuit).

Claims (1)

[Claims] (1) An envelope detection circuit that detects the envelope of the reproduced output, a conversion circuit that outputs at least a binary signal depending on the level of the output of this envelope detection circuit, and a change in the output state of this conversion circuit, A delay circuit that delays a change from at least one state to the other state, and outputs of the conversion circuit and the delay circuit are compared so that the conversion circuit output is in the other state and the delay circuit output is in the one state. A dropout detection circuit comprising a comparison circuit that outputs a dropout detection signal when .